The electrochemical technology is at the basis of several processes of industrial relevance, such as the production of aluminium from molten salts, of copper, zinc, nickel, cobalt and lead from aqueous solutions obtained by leaching of ores, of chlorine and caustic soda from sodium chloride solutions, of chlorine and optionally hydrogen from hydrochloric acid solutions and of hydrogen and oxygen from alkaline or acidic aqueous solutions.
In particular, in the case of chlorine-caustic soda electrolysis:2NaCl+2H2O→Cl2+H2+2NaOHthe industrial reactor known as electrolyser consists, in the technologically most advanced version of membrane electrolysis, of an assembly of elementary cells, each formed by two shells provided with perimetrical frames equipped with sealing gaskets, one membrane, two electrodes respectively known as anode and cathode secured to suitable supports and several nozzles for feeding the reactant-containing solutions and for discharging the products and the exhaust solutions. The membrane subdivides the internal volume of each elementary cell into two compartments, respectively the anodic one, containing the first of the two electrodes (anode) or current distributors (anodic) secured to the relevant supports, and the cathodic one containing the first of the two electrodes (cathode) or current distributors (cathodic) also secured to the relevant supports. Since the solutions, the reactants and the products present in the two compartments, being different (respectively sodium chloride solution and chlorine in the anodic compartment and caustic soda solution and hydrogen in the cathodic compartment) are characterised by a substantially different, chemical aggressiveness, the construction materials of the two shells cannot be the same: in particular the anodic shell is built with titanium sheet and the cathodic shell with nickel sheet.
When the single cells are assembled in electrical series to make up the electrolyser, the titanium anodic wall of one cell comes in contact with the nickel cathodic wall of the subsequent cell.
For a correct understanding of the content of the present invention, which will be illustrated in the following paragraphs, it is necessary to remind that from a constructional standpoint each elementary cell may be implemented as an independent unit consisting of a pair of shells, respectively cathodic and anodic, for instance as illustrated in DE 19816334 (constructive concept known to the experts in the field as “single cell”).
According to an alternative embodiment the elementary cell does not exist as independent unit, but is generated instead by the matching of suitable elements (known to the experts in the field as bipolar plates) when the electrolyser is assembled, as shown for instance in FIG. 3 of U.S. Pat. No. 4,767,519. In the case of chlorine-caustic soda electrolysis each bipolar plate comprises an anodic shell of titanium and a cathodic shell of nickel coupled in correspondence of the walls, for example by welding. During the assemblage of a similar kind of electrolyser the bipolar plates with the membranes and the perimetrical gaskets intercalated in-between are compressed one against each other: it is in this moment that the matching of the titanium anodic shell with the nickel cathodic shell of the subsequent bipolar plate forms the elementary cells.
In the case of alkaline water electrolysis:2H2O→2H2+O2 the two compartments, anodic and cathodic both contain a potassium hydroxide solution whose aggressiveness is not sensibly influenced by the presence of oxygen in the anodic compartment and of hydrogen in the cathodic one. Hence the two shells delimiting each elementary cell are usually made of the same material, nickel in particular, which is notoriously the metal characterised by best chemical resistance to alkaline solutions. This aspect has no effect at all in case the “single cell” design is adopted, while it brings to substantial constructional cost savings of the bipolar plates: in this case in fact the bipolar plate does not consist anymore of two mutually welded shells, as illustrated in the case of chlorine-caustic soda electrolysis, but rather of a single nickel sheet acting, once assembled the electrolyser, on one side as anodic wall delimiting the anodic compartment of one cell and on the other side as cathodic wall delimiting the cathodic compartment of the subsequent cell (single wall hereafter).
A totally equivalent situation occurs in the case of membrane water electrolysis, wherein the two solutions, anodic and cathodic, respectively consist of water and oxygen in the anodic compartment and of water and hydrogen in the cathodic compartment or, in a special process alternative, of water and oxygen in the anodic compartment and of humid hydrogen in the cathodic compartment. In both cases the construction material suited to both the anodic and cathodic conditions may be stainless steel, or preferably titanium considering its better chemical inertia.
A further situation, wherein the construction material compatible with both fluids, anodic and cathodic, may be the same, is given by hydrochloric acid electrolysis in its two variants, the conventional one with chlorine and hydrogen evolution:2HCl→H2+Cl2 or the oxygen cathode-depolarised one:4HCl+O2→2Cl2+2H2O
In both cases titanium and preferably some alloys thereof turn out to be compatible with the process conditions of both compartments, anodic (chlorine-containing hydrochloric acid solution) and cathodic (respectively hydrogen or oxygen with minor amounts of slightly acidic water). Thus, also in this case the bipolar plate may comprise a single sheet which during the electrolyser operation acts as anodic wall on one side and as cathodic wall on the other (see for instance U.S. Pat. No. 5,770,035).
A restraint to the adoption of the single wall bipolar plate design comes from the need of the two compartments, cathodic and anodic, to be provided with perimetrical flanges allowing to seal the same compartments, in co-operation with suitable gaskets, preventing the leakage of process fluids. In accordance with the known constructive procedures the perimetrical flanges are either obtained by moulding or by folding of the sheets constituting the compartment walls: in the case of bipolar plates comprising a pair of shells respectively destined to delimit an anodic compartment and a cathodic compartment, as happens in the case of the chlorine-caustic soda electrolysis, each wall is processed to form only its own flange and this operation gives in general satisfying results in terms of planarity and of absence of mechanical defects. Conversely in the case of bipolar plates comprising a single wall it is necessary to proceed, operating with the known procedures, to a double forming since both flanges, cathodic and anodic, are necessarily part of the same wall. The double forming, independently from the fact that it is accomplished by moulding or by folding, introduces high mechanical stresses in the material with remarkable distortions and/or frequent mechanical defects such as tears.
The problem is tackled in EP 1 366 212, where several types of perimetrical frames suitable for being used with bipolar plates comprising a single wall are described. In particular, it is proposed the use of a frame consisting of a bar or tube both with quadrangular profile, on a face of which the single wall of the bipolar element is welded, optionally in two subsequent passes in order to better ensure the absence of defects capable of releasing the process fluids to the external environment. In a further embodiment, the single wall is processed by moulding or folding so as to form a single flange, for instance the anodic flange, while the second flange, for instance the cathodic flange, is preassembled by moulding or folding of a suitable strip of identical material as that of the single wall: the preassembled cathodic flange is then secured by welding to the single wall already provided with the anodic flange. A further manufacturing alternative proposed as well in EP 1 366 212 provides that both flanges, anodic and cathodic, be preassembled in form of U-profiled frame obtained by moulding or folding of a suitable strip and that the single wall, completely planar, be secured, for instance by welding, in the middle of the frame. The problem of both manufacturing procedures is given by the need that the welds, whose linear development is remarkable, present no defect of a size such as to allow the outflow of the process fluids to the external environment. It follows that the procedure requires exasperated quality controls, moreover with the frequent intervention of the operators to eliminate the detected defects. All in all the construction turns out to be too expensive and not compatible with marketing requirements.
The above considered production cost aspect is further influenced by the securing procedures of the anodes and the cathodes, which comprise the production of the relevant supports and the execution of the required welds.
The reduction of the latter costs was considered in WO 03/038154, which proposes to obtain the supports for the anodes and cathodes directly from the single wall through the moulding of projections, for instance of prismatic shape, on both faces. The anodes and cathodes are subsequently secured on the top of the projections, for example by welding: the above described procedure allows a saving since the support material is given by the single wall and the required welds are reduced to those for securing the anodes and the cathodes on the tops of the projections getting rid of the wall to support welds, which are needed when the supports are preassembled separately, as customary in the conventional constructions. The drawback which prevents from advantageously using the manufacturing procedure of WO 03/038154 is given by the remarkable deformation undergone by the sheet of the single wall during the moulding of the projections on both faces, with the consequence of a totally unacceptable planarity for bipolar plates which have to be assembled in a high number to form the industrial electrolysers.
The survey of the prior art thus allows stating that the production of bipolar plates comprising a single wall, provided with anodic and cathodic flanges as well as of electrodes or current distributors with the relevant supports and characterised by suitable planarity and acceptable production costs on the commercial standpoint certainly represents an industrially relevant problem, with no viable solution so far.